The present invention relates generally to communication techniques and in particular to a spread spectrum communication technique over a shared asynchronous channel of a communication network.
A spread spectrum communication system is a system in which the transmitted signal is “spread” over a wide range of frequencies. Typically the bandwidth is much wider than the minimum bandwidth needed to transmit the information being sent. The underlying premise of this technique is that, in channels (typically wireless) with narrowband noise, an increase in the transmitted signal bandwidth produces a corresponding increase in the probability that the received information will be correct.
Though inefficient in its use of bandwidth, an advantage of spread spectrum is its resistance to interference. Another advantage is that the technique can be combined with existing systems having narrower operating bandwidths that fall within the spread spectrum bandwidth. The presence of a spread spectrum signal only slightly increases the noise floor that the narrow band receivers see.
Various spread spectrum techniques are known: In a frequency hopping system, the carrier frequency of the transmitter changes from among a pre-selected set of carrier frequencies in accordance with a pseudo-random code sequence. The frequencies selected from the list are dictated by the codes in the sequence. In a time hopping system, the period and duty cycle of the pulsed radio frequency (“RF”) carrier are varied in a pseudo-random manner in accordance with the pseudo-random code sequence. In a pulsed frequency modulated (“FM”) system, the RF carrier is modulated with a fixed period and fixed duty cycle sequence. During the transmission of each pulse, the carrier frequency is frequency modulated. Hybrid systems incorporate aspects of two or more other systems.
Direct sequence (“DS”) spread spectrum is a well known technique for transmitting digital data. The name direct sequence derives from the fact that the data sequence is directly multiplied by a high rate spreading sequence before it is transmitted over the channel. The spreading sequence is a sequence that transitions much faster than the data sequence. Instead of being called bits as in the data sequence, the individual states of the spreading sequence are called chips. The ratio of the chip rate to the data rate is commonly known as the spreading gain (or processing gain), since it is the ratio by which the bandwidth of the data sequence is increased once it is multiplied by the spreading sequence.
Refer to FIG. 1 for an illustration of the operation of direct sequence signaling. The figure shows a time domain representation of a case in which the chip rate of spreading sequence signal C is four times the rate of the data sequence D; i.e.
      1          t      C        =      4    ×                  1                  t          D                    .      The resulting spread signal S is the product of the two sequences. When the data bit in D is in a first logic state, signal S follows the spreading sequence C and when the data bit in D is in second logic state, signal S is the complement of C. The spreading sequence signal shown in FIG. 1 does not repeat. In practice though, many conventional direct sequence spread spectrum techniques employ a spreading code that repeats once per data bit. Code representation 100 is the notational convention used in the disclosure of the present invention to represent a spreading sequence in explaining the operation of the invention. The top half of representation 100 contains an identifier for the code, in this case “Code C.” The bottom half of representation 100 indicates the various chips in the spreading sequence and identifies them by number.
These spreading sequences are generally referred to as codes, as in Code Division Multiple Access (“CDMA”). CDMA generally refers to a technique by which users are allocated different spreading codes to enable them to use the same channel without interfering with each other. Another technique, dubbed Spread ALOHA CDMA (“SA/CDMA”), uses a single maximal length code (or a small number of maximal length codes for different service classes) that repeats once per data symbol and relies on arrival time to separate the different incoming signals. A maximal length code is a code of length 2M−1, where M is an integer, and which has certain desirable autocorrelation properties. For more information on maximal length codes, refer to Pateros, Charles N., “An Adaptive Correlator Receiver for Spread Spectrum Communication,” Ph. D. Thesis, Rensselaer Polytechnic Institute, Troy, N.Y., 1993.
Various code spreading strategies are known: In U.S. Pat. No. 5,084,900 to Taylor, the technique described uses a slotted Aloha CDMA system, where each packet uses the same code for the first transmission. When a collision occurs on the first packet, the packet is retransmitted using randomly selected codes from a known pool of codes. No particular code type or relationship of code length to symbol interval is specified.
In U.S. Pat. No. 5,450,395, Hostetter et al. describe a single code system where data bit length codes are used and each user always transmits a data ‘one’. The data is encoded in the time position of each broadcast. The technique requires accurate time synchronization among all of the users.
In U.S. Pat. No. 5,537,397, Abramson describes a spread ALOHA CDMA technique in which multiple users employ the same spreading code. The code is a maximal length code that repeats once per symbol. The users are time aligned at the chip level, but asynchronous at the data bit level. A subtractive multi-user receiver for this application is also described. In U.S. Pat. No. 5,745,485, Abramson extends his '397 patent to allow multiple codes to support different traffic types on the same channel.
Dankberg, et al. describe, in U.S. Pat. No. 5,596,439, a technique for self-interference cancellation for a relay channel. The technique is referred to as Paired Carrier Multiple Access (“PCMA”), since it allows a pair of channels (one in each direction) to share the same relay channel.
In U.S. Pat. No. 5,761,196, Ayerst, et al. describe a CDMA system employing re-use of spreading sequences. A central controller distributes seeds for maximal length codes as needed by users.
What is needed is a communication method which improves upon the multiple access collision performance of the prior art communication techniques. What is needed is a system which can realize improved performance over prior art CDMA communication systems without further complication to the system.